As is well known, an electric vehicle such as an electric automobile or a hybrid automobile is equipped with an AC rotating electric machine, as a driving source for the vehicle. An electric power converter connected with the AC rotating electric machine has a first electric-power conversion function for converting DC electric power from a DC power source into AC electric power to be supplied to the AC rotating electric machine and a second electric-power conversion function for converting AC electric power generated by the AC rotating electric machine into DC electric power with which the DC power source is charged. An electric-power conversion circuit formed of a switching device such as a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is provided in the electric power converter, so that the electric power converter realizes the foregoing electric-power conversion functions. In general, a switching device such as a MOSFET is broken due to a withstanding-voltage breakdown, when a voltage that is higher than a predetermined withstanding voltage is applied thereto. Accordingly, it is required to suppress the voltage to be applied to the switching device from exceeding the withstanding voltage of the switching device.
In recent years, mainly in Europe where gasoline mileage regulations are being strictly strengthened, there has been a technological trend that the voltage of the power source system of a vehicle is set to 48 [V] and hence the output voltage of an AC rotating electric machine, as an electric power generator, is raised so that gasoline mileage is improved without utilizing any expensive high-voltage hybrid system. When the power-source voltage is the same as or higher than 60 [V], safety measures are required in order to reduce the danger against a human body; however, because in the foregoing technological trend, the voltage of the power source system of a vehicle is suppressed to be the same as or lower than 60 [V], the cost for the safety measures can be reduced.
For example, in the case of a permanent-magnet synchronous rotating electric machine whose rotor is provided with magnetic-field poles that are each formed of a permanent magnet, the value of the induction voltage induced across the armature coil increases as the rotation speed rises. Accordingly, in the case of an electric vehicle equipped with a permanent-magnet synchronous motor, as a driving source for the vehicle, there is provided a voltage suppression apparatus for suppressing a voltage to be applied to a switching device provided in an electric power converter from exceeding the withstanding voltage of the switching device. For example, the conventional motor control apparatus disclosed in Patent Document 1 is configured in the following manner: when due to an induction voltage in the motor, the voltage to be applied to the switching device becomes the same as or higher than a predetermined voltage, all the upper-arm switching devices for respective phases, in the inverter circuit as an electric power converter, which are connected with the high-voltage side of the DC power source are turned on or all the lower-arm switching devices for respective phases, in the inverter circuit, which are connected with the low-voltage side of the DC power source are turned on; thus, the motor is made to be in the three-phase short-circuiting state; as a result, an electric current is made to circulate between the motor and the inverter as the electric power converter so as to suppress the voltage applied to the switching devices.